A typical vehicle air braking system includes an air compressor for supplying compressed air to the vehicle brakes. Compressed air from the compressor is directed through an air dryer to remove moisture and other impurities, then is stored in one or more supply or service reservoirs. When the vehicle brakes are actuated, compressed air is drawn from the supply or service reservoir to power the brakes.
The compressor operates on a cut-in and cut-out cycle. The compressor cuts in, that is, is actuated, when the pressure in the supply or service reservoir drops to a predetermined value, for example, 100 psi. When the pressure in the supply or service reservoir thereafter increases to a second predetermined level, for example, 130 psi, the compressor cuts out.
The air dryer is a desiccant which acts as a filter, removing and storing impurities, such as water, from the compressed air. When the compressor cuts out (is “unloaded”), a purge mechanism may be actuated to clean the air dryer of impurities it has collected. The purge mechanism includes a purge volume, which is a volume of compressed air that is preferably very clean, stored in a purge reservoir. The purge mechanism also includes a purge valve. To purge the air dryer, the purge valve is opened. When the purge valve is opened, the stored compressed air in the purge reservoir is enabled to flow, at a low pressure, through the air dryer, in a backflow manner. The backflow of clean, low pressure, air through the air dryer cleans, or purges, the impurities from the air dryer. The desiccant is regenerated. When this purge cycle is complete, the purge valve is closed, and the system returns to its previous operational state.
The purge reservoir is sometimes a second reservoir that is separate and distinct from the supply or service reservoir. The purge reservoir may, alternatively, be a portion of the supply or service reservoir that is kept separate by baffles in the reservoir. In either case, two fluid flow lines are provided from the air dryer, one for directing high pressure air to the supply or service reservoir, and one for directing high pressure air to and low pressure air from the purge reservoir to the air dryer.